Hi, I'm on Win 8.1 and have installed the DF trial on two machines however neither appears to detect a second monitor. How do I troubleshoot?

Do your machines detect the second monitor without DisplayFusion running?

These are two separate machines, each with one monitor.

If I misunderstood your reply then sorry in advance but the way I have read it is you have one machine with one monitor and another machine with one monitor. DisplayFusion works with your main or primary monitor and any other screens attached to the same computer. It cannot replicate a screen on one machine to a screen on another machine.